Cast aluminum bodies have found particular application in engine constructions because of their light weight and thermal conductivity. In applications of this type, good wear resistance is of considerable importance; the casting industry has turned to aluminum-silicon alloys which permit refining or precipitation of silicon as a primary phase to achieve said wear resistance. The prior art has appreciated that small and well dispersed particles of primary silicon in an aluminum-silicon eutectic matrix will improve wear resistance and other physical characteristics. Commercial refiners or modifiers have been developed to effect either refinement of primary or eutectic silicon, such as phosphorous or sodium. More recently, the art has appreciated that by the introduction of aluminum oxide to the casting melt, in a finally divided and uniformly dispersed condition, both primary and eutectic silicon can be provided in a precipitated form.
As desirable as the ultimate wear characteristics of an aluminum-silicon alloy may be, there are certain cost penalties inherent in producing such an alloy. Optimum costs can be achieved if a more simple aluminum material (less alloyed) is utilized while effecting some form of wear resistance at preferential selected surfaces of the casting where the latter is primarily required. The prior art is unable to provide and has not appreciated the benefits that can be obtained by providing a restricted zone of silicon with sufficient silicon particle surface area exposed for wear resistance and yet ultra-thin to insure adequate bonding of each particle to the aluminum substrate. Attempts by the prior art to provide a composite of metal powders adhered to a differential metal substrate has been by the use of the slurry technique. A slurry mixture of extremely fine powdered metal (such as nickel) is coated upon a mold cavity or other surface defining the mold cavity. The molten casting material is poured thereinto and cast in metallurgical relationship. This technique requires removal of water constituting the slurry. The extremely fine particle size of the metal powder in the slurry prohibits satisfactory wear resistance and good metallurgical bond.
There are other problems associated with the precipitation of silicon from the aluminum matrix in an aluminum-silicon alloy. A change in density is brought about by the presence of precipitated silicon and is due primarily to two phenomenon: (a) the solid solubility of silicon and aluminum and (b) its presence in a mixture. For silicon alloys containing 1.65% silicon or less (provided such material is given a solution heat treatment to insure that all of this silicon is in solid solution), the silicon in solution will decrease the lattice parameter of aluminum and therefore the density of the alloy will increase as a result of considerable shrinkage upon solidification. For silicon alloys containing in excess of 1.65% silicon, the latter will be out of solution and the density will be reduced by the rule of mixtures but shrinkage will still take place as a result of silicon that is in solid solution.
A typical commercial aluminum-silicon alloy for engine use is designated 390 and contains 16-18% silicon, 4-5% copper, 0.1% maximum manganese, 0.0-1.1% iron, 0.45-0.65% manganese, 0.1% maximum zinc, 0.2% maximum titanium, traces of phosphorous and the remainder aluminum. The refinement of the silicon particle size is controlled principally by the rate of cooling through the liquidus temperature range (which is approximately 1200.degree. F.). The coefficient of thermal expansion characteristic for the 390 alloy is essentially 12.0.degree. F. times 10.sup.-.sup.6 upon being heated from 68.degree. F. to 572.degree. F. This factor is in addition to the shrinkage characteristic which is the reverse of thermal expansion.